The present invention generally relates to fans and more particularly, to an impeller for a cross-flow fan which is widely used as a fan for an air-conditioner or the like.
Conventionally, in cross-flow fans, it has been so arranged as shown in FIG. 1 that a stabilizer 2 and a rear guider 3 are, respectively, disposed forwardly of and rearwardly of an impeller 1. The impeller 1 has a pair of disk-like end plates 4 disposed at opposite ends thereof and a plurality of disk-like or annular partition plates 5 interposed between the end plates 4. Adjacent ones of the end plates 4 and the partition plates 5 are spaced at a predetermined interval so as to be coupled, along outer peripheral portions thereof, with each other by a number of blades 6.
Since the cross-flow fans generally have such structural features as to produce an air flow incapable of being achieved by fans of other types, that, is an inflow direction and an outflow direction of air which are opposite to each other as shown by the arrows of FIG. 1, and an air flow which increases substantially in proportion to lengths of the cross-flow fans upon axial extension thereof, the cross-flow fans have come into general use for air-conditioners, etc. in recent years.
However, the known cross-flow fans have such disadvantages that high frequency noises are produced and their efficiency is rather low. Regarding the noises, the prior art cross-flow fans have such an inconvenience that highfrequency peculiar noises, i.e. so-called nz noises are readily generated, which are produced by interference between the stabilizer 2 and the blades 6 of the impeller 1 and whose frequency is expressed by a product of n.multidot.z (n and z denoting the rate of revolutions of the impeller 1 and the number of the blades 6, respectively). Although the nz noises are peculiar noises experienced widely also in fans of other types, frequencies of the nz noises rise especially in the cross-flow fans so high as to be extremely disturbing for human ears like whistling sounds.
Although the pitch of the nz noises varies according to shapes or relative positions of the impeller 1, stabilizer 2 and rear guider 3, loads, etc., the design for repressing the nz noises frequently runs counter to the design for raising the fan efficiency. For example, it is already well known that a minimum distance y (FIG. 1) between the stabilizer 2 and the impeller 1 is an essential factor for determining the performance of the cross-flow fans. In the case where the minimum distance y is reduced, the efficiency rises but the nz noises become undesirably large. Thus, the known cross-flow fans have such a drawback that, in order to repress the nz noises, the efficiency cannot be raised as high as might be desired.
Furthermore, in the prior art impeller 1, since the blades 6 are arranged at a uniform pitch angle .theta. (=360/z deg.) as shown in FIG. 2, interference noises due to interference between the blades 6 and the stabilizer 2 are produced at a uniform interval as shown in FIG. 4a, so that the interference noises produced by all the blades 6 have an identical frequency characteristic and thus, the noise level becomes extremely high at a frequency of nz.